Yogurt Monograph Series: Sensory Properties
Daniel Wilbanks, Technical Contributor
Desirable sensory properties for yogurt varies between styles of yogurt and with the preference of consumers. For instance, additional sugar is often - though not always - included with stirred style yogurts, while skyr is known more for its thick texture and sour taste and so contains little or no added sugar. Additionally, consumer preferences change over time, which makes universal rules for proper yogurt sensory properties impossible to codify. Instead, we will turn our focus here to the factors that produce the unique flavors and textures one may find in various yogurt varieties.
Perhaps the most defining characteristic of yogurt is its acid flavor. The most common end point for fermentation is pH ~4.6, which provides a pleasant acid flavor and helps to preserve the food from most microorganism-based spoilage. While cooling yogurt to refrigeration temperature slows the fermentation process, it does not stop it completely and acidification continues after packaging (post-acidification) unless the yogurt is heated to inactive the microbes. Post-acidification is typically considered to be a defect as it leads to additional sour flavor and more importantly can lead to syneresis. Many modern cultures are selected for their poor growth beyond a particular acid level to limit post-acidification.
Additional flavor compounds may be produced by yogurt starter cultures (L. bulgaricus and S. thermophilus) but adjunct cultures are often used to impart additional flavors (Table 1). For instance, acetaldehyde, a major flavor compound produced by lactic acid bacteria in yogurt, is described as having a green apple flavor and odor. Many flavor compounds are volatile and degrade over time or may be converted by microbes to other end products. Sweetness is an attribute in yogurt that is important but not always desirable. US consumers generally prefer moderate-to-high sweetness scores in their yogurt but are increasingly wary about high levels of added sugar. Recent changes to FDA nutritional labeling requirements now require added sugar to be listed separately under total carbohydrates to draw attention to products with added sugar. Artificial sweeteners, such as acesulfame K and saccharin, have been used in the past to supplement or replace sugar but are becoming rare in low calorie yogurt options, mostly due to widespread consumer acceptance of stevia, a natural sugar substitute.
The primary sugar in milk, lactose, is a disaccharide comprised of 2 sugar monomers (saccharides) - glucose and galactose. As sweetness is concerned, lactose is an
example of a sugar that is not the sum of its parts; that is, glucose and galactose are individually sweeter than they are when bonded together to form lactose (Table 2). By hydrolyzing the glycosidic bond in lactose with enzymes, it is possible for yogurt manufacturers to break lactose apart into glucose and galactose monomers
prior to, or during, fermentation which increases the sweetness of the finished product without the addition of sweeteners. The complete hydrolysis of lactose also allows manufacturers to label yogurt as lactose free.
Flavor additives are common in yogurts and may be added as fruit
chunks, fruit puree, and/or flavor extracts before or after
fermentation. Care must be taken when incorporating fruit after heat
treatment of the milk as it has the potential to introduce yeast and
mold contamination, which are the primary microorganisms responsible for
spoilage in yogurt.
Texture is a critical sensory attribute that
can affect the overall acceptance of yogurt. Smooth texture is desired
for yogurt, with minimal lumps or grains present. Generally, higher
protein levels lead to thicker yogurt and higher fat levels lead to a
creamier yogurt. Thick and creamy texture are often conflated, but they
are in fact separate properties that are largely independent of each
other. Texture is covered in greater detail in its own Primer Article (Part 2).
This article originally appeared in the Fall 2023 issue of the Dairy Pipeline.